With each signal wave generally present for an interval spanning a multiplicity of cycles, the number of zero crossings of that wave within a given time interval is a measure of its frequency. Thus it is known to derive from such a wave, by differentiation, a train of pulses which can be converted into a binary wave of fixed pulse width or pulse spacing yielding, upon integration in a low-pass filter, an analog voltage alternating between two amplitude levels which represent a measure of the frequencies of the original signal waves upon being compared with a reference voltage.
In such a system, the output of the comparison circuit can be affected by fortuitous changes in the magnitude of the reference voltage. Thus, since the transitions of the analog voltage from one level to the other are gradual, such a shift in reference voltage may result in a distortion of the received message. The problem is aggravated, of course, in the presence of more than two signal frequencies.